KnowledgeQuest - Resistors

RESISTORS AND THEIR USES

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INTRODUCTION

The resistor is probably the most common and well known of
all electrical components. Their uses are many, they are
used to drop voltage, limit current, attenuate signals,
act as heaters, act as fuses, furnish electrical loads and
divide voltages.

These uses are basic, for example, the voltage divider
use is used in a variety of networks to divide voltages in
specified increments of the applied voltage such as for
analog to digital converters and digital to analog
converters. They are used as matched pairs with relative
accuracy much greater than their absolute accuracy.
Matching is used in building voltage dividers and Wheatstone
& Kelvin Bridges with extremely precision accuracy over a
wide range of temperatures. This is done by matching the
absolute value and the temperature coefficient of Resistance
(TCR). This accuracy is limited only by the ability to
accurately measure them and their stability.

There are numerous varieties of resistors. There are
Precision Wirewound., NIST Standards , Power Wirewound ,
Fuse Resistors, Carbon Composition, Carbon Film, Metal Film,
Foil, Filament Wound, , and Power Film Resistors. Each of
these resistors has a useful purpose.

Resistors have numerous characteristics which determine
their accuracy when used. Each will effect the accuracy to
a greater or lesser extent depending on the application.
Some of these characteristics are, Tolerance at DC,
Temperature Coefficient of Resistance (TCR), Frequency
Response, Voltage Coefficient, Noise, Stability with Time
and Load, Temperature Rating, Power Rating, Physical Size,
Mounting Characteristics, Thermocouple Effect, and
Reliability.

I will go into further details on type of resistor,
characteristics , and materials to manufacture them in
future articles. Most of my experience has been in the
design and manufacture of Bridges, Networks, Precision
Wirewound, Metal Clad Power and Power Wirewound. These
will be covered in greater detail..

These articles are intended to be general in nature. I
would recommend that the appropriate manufacturer be
consulted for specific characteristics on the resistors that
they manufacture. Each manufacturer will have a specific
group of characteristics in which they excel.

RESISTOR TERMS AND ABBREVIATIONS

Resistor Tolerance

Resistor Tolerance is expressed as
the deviation from nominal value in percent and is measured
at 25oC only with no appreciable load applied. It will
change depending on the other conditions when in use. For
example, a 100 ohm resistor with a tolerance of 10% can
range in value from 90 ohms to 110 ohms and this will change as
power is applied and the temperature varies.

Temperature Coefficient of Resistance

The Temperature
Coefficient of Resistance (TCR) is expressed as the change
in resistance in ppm ( .0001%) with each degree of change
in temperature Celsius (Co). This change is not linear
with the TCR the lowest at +25oC and increasing as the
temperature increases ( or decreases). It can be either a
bell shaped curve or an S shaped curve. It is treated as
being linear unless very accurate measurements are needed,
then a temperature correction chart is used. Normally a
resistor with a TCR of 100 ppm will change 0.1% over a 10
degree change and 1% over a 100 degree change. The
expression of ppm , one part in a million is similar to
percent or 1 part in 100 (or percentile)

Frequency Response

Frequency Response is the change in
resistance with changes in frequency and is more difficult
to measure. Where exact values are needed, these changes
can be plotted but not very accurately, and normally in db
change. These measurements can be made with a Boonton RX
Meter which is designed for measuring low Q circuits.

Noise

Noise levels are measured with very specialized
equipment. It is extremely difficult to measure accurately
and does not effect the value of the resistor but can have
a devastating effect on low signals, digital amplifiers,
high gain amplifiers, and other applications sensitive to
noise. The best approach is to use resistor types with low
or no noise in applications that are sensitive to noise.

Voltage Coefficient

The Voltage Coefficient is the change
in resistance with applied voltage and is associated with
Carbon Composition Resistors and Carbon Film Resistors. It
is a function of value and the composition of the carbon
mixture used in the manufacture of these resistors. This
is entirely different and in addition to the effects of self
heating when power is applied.

Thermocouple Effect

The Thermocouple Effect is due to the
Thermal emf generated by the change in the temperature at
the junction of two dissimilar metals. This emf is due to
the materials used in the leads or in the case of Wirewound
Resistors the resistive element also. This can be minimized
by keeping both leads at the same temperature. The thermal
emf is the result of the difference in the temperature of
one lead to the other lead. One lead will cause a positive
emf and the opposite lead will generate a negative emf ( or
visa versa). When both leads are at the same temperature,
the emf's generated will cancel each other and the same is
true where the resistive element joins the leads. Resistors
with nickel leads as used in certain welded module
applications will generate the highest thermal emf. The
resistive element (the wire) of wirewound resistors is
designed with a low thermal emf, but some of the wire used
for high TCR resistors will have a much larger thermal emf.

Stability

Stability is the change in resistance with time
at a specific load, humidity level, stress, and ambient
temperature. The lower the load and the closer to +25oC the
resistor is maintained, the better the stability. Humidity
will cause the insulation of the resistor to swell applying
pressure (stress) to the resistive element causing a change.
Changes in temperature alternately apply and relieve
stresses on the resistive element thus causing changes in
resistance. The wider the temperature changes and the more
rapid these changes in are, the greater the change in
resistance. If severe enough, it can literally destroy the
resistor. Rapidly and continuously subjecting a device to
it's lowest and highest operating temperatures(called a
Thermocycle Test) is considered a destructive test.

Reliability

Reliability is the degree of probability that
a resistor (or any other device) will perform it's desired
function. There are two ways of defining Reliability. One
is Mean Time Between Failures (MTBF) and the other is
Failure Rate per 1,000 hours of operation. Both of these
means of evaluating reliability must be determined with a
specific group of tests and a definition of what is the end
of life for a device, such as a maximum change in resistance
or a catastrophic failure (short or open). Various
statistical studies are used at arriving at these failure
rates and large samples are tested at the maximum rated
temperature with rated load for up to 10,000 hours (24 hrs
per day for approximately 13 months).

Temperature Rating

Temperature rating is the maximum
allowable temperature that the resistor may be used. There
are generally two temperatures for example, a resistor may
be rated at full load up to +85oC derated to no load at
+145oC. This means that with certain allowable changes in
resistance over life the resistor may be operated at +85oC
with it's rated power. It also may be operated with
temperatures in excess of +85oC if the load is reduced,
but in no case should the temperature exceed the design
temperature of +145oC with a combination of ambient
temperature and self heating due to the applied load.
A word of caution, some rated loads are at +25oC and must
be derated if the ambient temperature exceeds +25oC.

Power Rating

Power ratings are based on physical size,
allowable change in resistance over life. thermal
conductivity of materials, insulating and resistive materials,
ambient operating conditions. Again note that all resistors
are not rated alike. The safest bet is to use the largest
physical size and never use it at it's maximum ratings both
in temperature and power unless you are prepared to accept
the maximum allowable changes in resistance in life.
Another thing to note; the majority of change under those
conditions will occur during the first 100 hours of
operation.

It is important that all of the above characteristics be
considered when selecting a particular style and tolerance
for each application.

TYPES OF MOUNTING AND PHYSICAL SIZES.

RESISTOR SIZES

Resistors are available in almost any size
ranging from 0.065 inches diameter by .125 inches long to
12 inches in Diameter to several feet high (for very high
voltage resistors). They come in almost any shape that is
imaginable. The most common form is cylindrical with
leads coming out either end. They can be manufacture in
custom shapes to fit the available space when quantities
justify.

TYPE OF MOUNTING.

Resistors can be made with almost any
type of mounting. If the need arises, special mountings can
be designed to fit the customer's needs. Some of the more
common means of mountings are listed below. The term
"Leads" is used in the general sense as a means of
connecting the resistor. They may be lugs, wire leads,
pins, or any means of connecting the resistor to the
circuit.

Surface Mount

Resistors are available in a surface
mounting configuration. This is generally associated with
chip resistors that are mounted by solder reflow techniques.
This consists of a resistive element of a flat ceramic
substrate (or a cylindrical ceramic core) with a solder pad
on each end. Sizes range from .163 inches diameter to .555
inches long cylindrical to a .020 high by .031 wide by .062
long chip.

Fuse Clip Mounting.

The fuse clip type is made such that
it will mount directly into a fuse clip. Fuse resistors
are sometimes made like this.

Single Inline Packaging (SIP)

The Single Inline Package
is normally associated with resistor networks consisting of
several resistors in the same package. It is a rectangular
flat shaped package with the several leads coming out one
surface generally the narrow, long surface.

Dual Inline Package (DIP)

The Dual Inline Package is
again normally associated with resistor networks. The main
difference is that leads extend out both narrow, long
surfaces and are formed to either flush mount on a PC Board
or thru hole mount on a PC board.

Flat Packs

The Flat Pack are roughly the same as Dual
Inline Packaging except the leads come straight out and are
not formed for surface mounting or thru hole mounting. This
is just a variation of DIP mounting.

Axial Leads - Axial Lead mounting is what most of us are
familiar with using. It consists of a cylindrical (or
rectangular or any shape body) with the leads extending out
either end parallel to the resistor's major axis.

Radial Leads

Radial Lead mounting is similar to Axial
Lead mounting except the lead come out of the body
perpendicular to its major axis.

PC Mounting

PC mounting consists of both leads of the
resistor coming out the same surface so that it is easier
to mount a resistor (or any other device) vertically. The
resistor may be rectangular or cylindrical.

4 Terminal Mounting

Most styles will offer a 4 terminal
means of mounting for low values. This is important when
the leads become a significant part of the value. This
establishes the point on the leads where the value in
within the desired tolerance. It is fixed and prevents
changes in the value due to mounting variations.

RESISTOR TYPES

PRECISION WIREWOUND

The Precision Wirewound is a highly
accurate resistor with a very low TCR and can be accurate
within .005%. A temperature coefficient of resistance (TCR)
of as little a 3 part per million per degree Celsius
(3ppm/oC) can be achieved. However these components are
too expensive for general use and are normally used in
highly accurate DC applications. The frequency response of
this type is not good. When used in an rf application all
Precision Wirewound Resistors will have a low Q resonant
frequency. The power handling capability is very small.
These are generally used in highly accurate DC measuring
equipment, and reference resistors for voltage regulators
and decoding networks.

The accuracy is maintained at 25oC(degrees Celsius) and
will change with temperature. The maximum value available
is dependent upon physical size and is much lower than most
other types of resistor. Their power rating is approximately
1/10 of a similar physical size in a carbon composition.
They are rated for operation at +85oC or +125oC with
maximum operating temperature not to exceed +145oC. This
means that full rated power can be applied at +85 ( 125) oC
with no degradation in performance. It may be operated
above +125 (85) oC if the load is reduced. The derating
is linear, rated load at +125(85) oC and no load at +145oC.
Life is generally rated for 10,000 hours at rated
temperature and rated load. The allowable change in
resistance under these conditions is 0.10%. Extended
life can be achieved if operated at lower temperatures and
reduced power levels. End of life requirements are
generally defined by the manufacturer or in some case by
user specification. Some degradation in performance can be
expected. In some cases, particularly if the tolerance is
very low and the TC is low, the rated power is reduced to
improve resistor stability through life. Precision
Resistors regardless of type, are designed for maximum
accuracy and not to carry power. The materials used in
these resistors are highly stable heat treated materials
that do change under extended heat and mechanical stress.
The manufacturing processes are designed to remove any
stresses induced during manufacture.

There is little detectable noise in this type of resistor.
The stability and reliability of these resistors is very
good and their accuracy can be enhanced by matching the
absolute value and the temperature coefficient over their
operating range to achieve very accurate voltage division.

NIST STANDARD

The NIST (National Institute of Standards
and Technology) Standard can be as accurate as .001% with
roughly the same TCR as Precision Wirewound Resistors and
are very stable. These are used as a standard in verifying
the accuracy of resistive measuring devices. They are
normally the Primary Standards of a company's test lab.

They are returned to the NIST for measurement and their
accuracy is tracked through out the standards life to
determine the Standard's stability. Most companies will
have two sets of standards so that they can continue to
measure while one set of standards are being measured by
the NIST . They will alternate returning these NIST
Standards to the NIST , one set one year and the other set
the next year. For extremely accurate measurements,
the Standard with the longest history and the best
stability will be used. If erratic readings are received
from the NIST over a period of years, the Standard is
retired. Also, if the reading has significantly changed
since the last NIST reading, the standard is suspect and
all measurements made using that standard must be checked.

Normally, a standard will take about 3 years to stabilize
and becomes morestable with time unless it has had
excessive power applied or has been dropped. These
standards are generally stored in an oil bath at +25oC.
During measurement, a thermometer is placed in a cavity in
the top of the Standard, called the oil well, and the
temperature is recorded for each measurement so that the
exact value can be determined. That is the value at +25oC
plus or minus the change in value caused by the temperature
coefficient. Each standard will have a temperature
correction chart for exact values. Being stored in the
oil bath prevents the Standard from being stressed by
changes in room temperature. These are highly precision
devices and are expensive to buy and expensive to maintain,
but they are the primary resistor reference for any test
lab.

These resistors are furnished in a totally enclosed metal
case and for values above 1 ohm, this enclosure is filled
with mineral oil (other type of oil may contain additives
that can cause corrosion in later life). The values below
1 ohm may be built in an enclosure that is perforated and
these must be submersed in oil. If power is applied without
it being submersed, the Standard will be ruined.

All NIST Type Standards are equipped with provisions for
two, three, or four terminal measurements. The applied power
is calculated and the temperature of the Standard is
monitored during test. The lowest power level consistent
with sufficient resolution to get the desired measurement
is used (in the area of 0.01 watts) and any appreciable
rise in temperature will dictate that the measurement should
be suspended and the test set-up reviewed for ways to
reduce the power level. These Standards are rated for
operation at room temperature only but their other
characteristics are the same as Precision Wirewound
Resistors.

POWER WIREWOUND RESISTORS

Power Wirewound Resistors are used when it is
necessary to handle a lot of power. They will handle more power per unit
volume than
any other resistor. Some of these resistors are free wound similar to
heater elements.
These require some form of cooling in order to handle any appreciable amount
of power.
Some are cooled by fans and others are immersed in various types of liquid
ranging from
mineral oil to high density silicone liquids. Most are wound on some type
of winding
form. These winding forms vary. Some examples are ceramic tubes, ceramic
rods,
heavily anodized aluminum, fiberglass mandrels, etc.

To achieve the maximum power rating in the smallest package size, the core
on which the
windings are made must have a material with high heat conductivity. It
may be Steatite,
Alumina, Beryllium Oxide, or in some cases hard anodized Aluminum.
Theoretically, the
anodized Aluminum core has a better heat conductivity than any other
insulated material,
with Beryllium Oxide being very close. There are specific problems with the
anodized
aluminum cores such as nicks in the coating, abrasion during capping and
controlling the
anodized thickness. There are various shapes, oval, flat, cylindrical, and
most shapes are
designed to optimize heat dissipation. The more heat that can be radiated
from the
resistor, the more power that can be safely applied.

There is a group of these called "Chassis Mounted Resistors". These are
generally
cylindrical power resistors wound on a ceramic core molded and pressed into
an aluminum
heat sink and usually with heat radiating fins. These are designed to be
mounted to metal
plates or a chassis to further conduct heat. This result in a rating
approximately 5 times or
more its normal rating.

These resistors come in a variety of accuracy's and TCRs. They can be
custom made as a
cross breed between a Precision Resistor and a Power Resistor; capable of
handling more
power than the standard Precision Wirewound but not as accurate. Practically
speaking,
tolerances of 1% and temperature coefficients of 20 ppm can be achieved on
all except the
parts that are coated with Vitreous Enamel and low values. The curing
process for
Vitreous (a type of glass) requires extremely high heat and shrinks applying
pressure to
the winding. This particular group normally will run tolerances of 10% with
a TCR of
100ppm/oC. Power Resistors come in a variety of ratings. Most are rated at
+25oC and
derated linearly to either +275oC or +350oC. Again if the ambient
temperature of
operation is +275oC, no power can be applied and at +125ooC 1/2 rated power
can be
applied.

These power rating are based on mounting the resistor in free air with the
leads terminated
at the recommended point. On axial lead components, this is 3/8 of an inch
from the body.
If they can be mounted closer, the resistor will run cooler or you can apply
slightly more
power and if mounted further out, you must reduce the power. CAUTION, if
mounted
directly over and in contact with a printed circuit board, the heat from the
resistor can char
the board if full power is applied. I don't know of any PC Boards that are
rated at
+275oC.

Other means of increasing the amount of power you can apply

(a) bond the resistor to the chassis or other metal parts

(b) mount vertically to get the chimney effect (this is very helpful when
using those
wound ceramic tubes)

(c) terminate as close to the body as practical

(d) submerse in oil (CAUTION some types of resistor coating,
particularly silicone
based coatings will disintegrate when immersed in oil and heated).
This will increase
the rating as much as 5 times. or reduce the temperature rise of
the resistor due to
self heating.

The small power resistor can serve a two fold purpose, that is to fulfill
it's purpose as a
resistor and act as a heater in an enclosure. Some users have used them in
crystal ovens to
maintain the crystal at the desired temperature. It makes a reasonably
cheap off the shelf
heater that comes in a variety of wattage's , sizes and values.

One unique type of power resistor is the "Bathtub Boat Type". This consists
of resistance
wire wound on a fiberglass cord.. This is a continuously wound strip, cut
into strips of
the appropriate length with leads crimped. These resistive elements are
placed in a
ceramic shell (boat) and an highly filled cement is used to fasten these in
the boat. The
filler often used in the cement is a ceramic material with high heat
conductivity. These are
very inexpensive, no effort is made to achieve tight tolerances, low TCRs,
and the range
of values is extremely limited. They are often found as surge resistors in
TVs and other
electronic /electrical equipment. Their main selling point is low cost.
They are often sold
with an enamel coating for a low power precision wirewound resistor that is
even lower in
cost.

One more item to consider, Power Wirewounds are made using alloys with melt
temperatures ranging from +1200o C to +1500o C and may be operated cherry red
without failure for short periods of time, however the resistance value and
TCR will
change significantly and the insulating material will severely degrade. The
bathtub boat
type can not be subjected to this type of overload, the fiberglass winding
form will
disintegrate.

FUSE RESISTORS

Fuse Resistors serve a dual purpose, a resistor and a
fuse. They are
designed so that they will open with a large surge current. The fusing
current is calculated
based on the amount of energy required to melt the resistive material (the melt
temperature plus the amount of energy required to vaporize the resistive
material).

These resistors will normally run hotter than a normal precision or power
resistor so that a
momentary surge will bring the resistive element up to fusing temperature.
Some designs
create a hot spot inside the resistor to assist in this fusing.
Calculations are made and
samples are produced to verify the calculations. The major unknown is the
heat transfer
of the materials, which can be quite significant for pulse of long duration,
and is very
difficult to calculate.

Mounting of these devices is critical because it will effect the fusing
current. These are
quite often made to mount in fuse clips for more accurate fusing
characteristics.

CARBON COMPOSITION

Carbon composition resistors were once the most common
resistor on the market. They still have a very large market and prices are
highly
competitive. They are made from carbon rods cut in the appropriate length
then molded
with leads attached. The mix of the carbon can be varied to change the
resistivity for the
desired values.

High values are much more readily available. Very low values are more
difficult to
achieve. A 5% tolerance is available. This is usually done by measuring
and selecting
values. Normal tolerances without measurement and selection is in the area
of 20%.

The temperature coefficient of resistance is in the range of 1000 ppm/oC and
is negative,
that is when the temperature goes up the resistance goes down and when the
temperature
goes down, the resistance goes up. This is due to the carbon particles
being relaxed (with
increase in temperature) and being compressed (with the reduction in
temperature).

These resistors also has a voltage coefficient. That is the resistance will
change with
applied voltage, the greater the voltage, the greater the change. In
addition to a power
rating, they also have a voltage rating. (The wirewound voltage rating is
determined by
the value and the wattage rating). The voltage rating of Carbon Composition
Resistors is
determined by physical size as well as the value and wattage rating.

One more item to consider is that due to their construction, they generate
noise and this
noise level varies with value and physical size. The power capability in
relation to physical
size is greater than Precision Wirewounds but less than Power Wirewounds.

CARBON FILM RESISTORS

Carbon Film Resistors have many of the same
characteristics as carbon composition resistors. The material is similar
therefore they
have noise, a voltage coefficient, the TCR can be much lower because the
formula can be
varied to achieve this, the tolerance is much tighter due to the difference
in manufacturing
processes.

The Carbon Film Resistor is made by coating ceramic rods with a mixture of
carbon
materials. This material is applied to these rods in a variety of means,
the one most
familiar to me are dipping, rolling, printing , or spraying the rods in the
appropriate
solution. The thickness of the coating can be determined by the viscosity
of the solution.
This as well as the material composition will determine the ohms / square.
Some of you
may not be familiar with this term. It simply means that if a material has
a resistivity of
100 ohms / square, one square inch with have the same resistance as 1 square
mm, or 1
square foot or 1 square yard or 1 square mile all equaling 100 ohms but the
power
handling capability is proportional to the size.

One batch of material can produce resistors in a wide range of values.
These rods are cut
to the length required for a specific size of resistor. These rods can then
be spiral cut to a
wide range of values. The original method of spiraling these was done with
grinding
wheels on a machine similar to a lathe. I am sure that later processes use
lasers that are
programmed to cut to specific values. The maximum ohmic value of this group
is the
highest in the descrete resistor group.

Tolerance of 1% can be achieved with out measuring and selecting. Tolerance
of less than
1% can be achieved by measuring and selecting. You should use caution in
getting tight
tolerances in this type because the temperature coefficient, voltage
coefficient and stability
may mean that it is only good for that tolerance at the time it was
installed. The TCR of
carbon film resistors is in the neighborhood of 100 to 200 ppm and is
generally negative.
Measuring and selecting can yield even tighter TCRs.

The frequency response of this type of resistor is among the best, far
better than
Wirewounds, and much better that carbon composition. The wirewound
resistors are
inductive at lower frequencies and values and somewhat capacitive at higher
frequencies
regardless of value. Also wirewound resistors will have a resonant
frequency. Carbon
Composition Resistors will be predominately capacitive .

METAL FILM RESISTORS

Metal Film resistors are the best compromise of all
resistors. They are not as accurate and have a higher temperature
coefficient of resistance
and are not as stable as Precision Wirewounds. They are more accurate, do
not have a
voltage coefficient, have a lower temperature coefficient than Carbon Film.
TCRs of 50
to 100 ppm can be achieved.

They have a very low noise level when properly manufactured. In fact some
of the
screening processes measure the noise level to determine if there are
problems in a
particular batch of resistors.

Metal film resistors are manufactured by an evaporation/deposition process.
That is the
base metal is vaporized in a vacuum and deposited on a ceramic rod or wafer.
Several
attempts have been made to vaporize low TCR materials and deposit on these
substrates,
but to my knowledge, these attempts have not been successful. This is
partially due to the
different boiling points of the various base metals in these alloys (I use
the word alloy not
entirely accurately, for these materials are not true alloys but
amalgamations --- they do
not bond to form a molecule as does a true alloy). The very low TCR
resistive materials
are heat treated to achieve the resistivity and low TCR. This is not
compatible with an
evaporation process.

The frequency characteristics of this type are excellent and better than
Carbon Films. The
one area that carbon films exceed metal films is the maximum values. Carbon
films can
achieve higher maximum values than any other group.

FOIL RESISTORS

Foil resistors are similar in characteristics as metal
films. Their main
advantages are better stability than metal films and lower TCRs. They have
excellent
frequency response, low TCR, good stability, and very accurate. They are
manufactured
by rolling the same wire materials as used in precision wirewound resistors
to make thin
strips of foil. This foil is then bonded to a ceramic substrate and etched
to produce the
value required. They can be trimmed further by abrasive processes, chemical
machining or
heat treating to achieve the desired tolerance. Their main disadvantage is
the maximum
value is less than Metal Film Resistors.

The accuracy is about the same as metal film resistors, the TCR and
stability approaches
Precision Wirewounds but somewhat less because the rolling process and the
packaging
process produce stresses in the foil. The resistive materials used in
Precision Wirewound
Resistors is very sensitive to stresses which result in instability and
higher TCRs. Any
stresses on these material will result in a change in the resistance value
and TCR, the
greater the stress, the larger the change. This type can be used as strain
gauges, strain
being measured as a change in the resistance. When used as a strain gauge,
the foil is
bonded to a flexible substrate that can be mounted on a part where the
stress is to be
measured.

FILAMENT RESISTORS

The Filament Resistors are similar to the Bathtub Boat
Resistor except they are not packaged in a ceramic shell (boat). The
individual resistive
element with the leads already crimped is coated with an insulating
material, generally a
high temperature varnish. These are used in applications where tolerance,
TCR, and
stability are not important but the cost is the governing consideration.
The cost on this
type is slightly higher that carbon composition and the electrical
characteristics are better.

POWER FILM RESISTORS

Power film resistors are similar in manufacture to their
respective metal film or carbon film resistors. They are manufactured and
rated as power
resistors, with the power rating being the most important characteristic.
Power Film
Resistors are available in higher maximum values than the Power Wirewound
Resistors
and have a very good frequency response. They are generally used in
applications
requiring good frequency response and/or higher maximum values. Generally
for power
applications, the tolerance is wider, the temperature rating is changed so
that under full
load resistor will not exceed the maximum design temperature, and the
physical sizes are
larger, and in some cases, the core may be made from a higher heat
conductive material
and other means to help radiate heat.